1. Field of the Invention
The present invention relates to a light emitting device having a circuit constituted of thin film transistors (hereinafter referred to as TFTs) and a method of manufacturing the same. Note that the light emitting device in this specification refers to a device such as an image display device having a light emitting element in which light emission is obtained by applying an electric field. Further, a module in which a light emitting element is attached with a connector, for example, an anisotropic conductive film ((FPC: flexible printed circuit) or a TAB (tape automated bonding) tape, or a TCP (tape carrier package), a module in which a printed wiring board is provided at an end of a TAB tape or TCP, and a module in which an IC (integrated circuit) is directly mounted on a light emitting element by a COG (chip on glass) method are all included in the light emitting devices. In addition, electric equipment, which is mounted with the light emitting device as a part, are also included.
2. Description of the Related Art
In recent years, a technique for forming a TFT on a substrate has been greatly progressed, and application and development to an active matrix display device has been progressed. Especially, since a TFT using a polysilicon film has an electric field effect mobility (also called mobility) higher than that of a TFT using a conventional amorphous silicon film, a high speed operation is possible. Thus, control of a pixel, which is conventionally performed by a driver circuit outside a substrate, can be performed by a driver circuit formed on the same substrate as the pixel.
In such an active matrix display device, various merits such as reduction of manufacturing cost, miniaturization of an electro-optical device, improvement of a yield, and reduction of a throughput can be obtained by forming various circuits and elements on the same substrate.
Further, research of an active matrix light emitting device including an EL element as a self light emitting element has been actively carried out.
Note that, an EL element in this specification has a structure in which an EL layer is sandwiched between a pair of electrodes (anode and cathode), and the EL layer generally has a lamination structure. Typically, there is cited a lamination structure called “hole transporting layer/light emitting layer/electron transporting layer” proposed by Tang et al. of Eastman Kodak Company. This structure has a very high luminous efficiency, and most of the light emitting devices on which research and development have been made at present adopt this structure.
In addition, a lamination structure in which hole injecting layer/hole transporting layer/light emitting layer/electron transporting layer or hole injecting layer/hole transporting layer/light emitting layer/electron transporting layer/electron injecting layer are laminated in the order on an anode may be taken. The light emitting layer may be doped with a fluorescent pigment or the like.
In this specification, all the layers provided between a cathode and an anode are generically called EL layers. Thus, the hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer, and electron injecting layer described above are all included in the EL layers.
Then, the EL layer with the above structure is applied with a predetermined voltage from a pair of electrodes, whereby recombination of carriers is generated in the light emitting layer to obtain light emission. Note that the light emission obtained here includes fluorescence and phosphorescence. Further, a light emitting element composed of an anode, an EL layer and a cathode is referred to the EL element in this specification.
Deterioration of the EL layer of the EL element is promoted by heat, light, moisture, oxygen or the like. Thus, the EL element is generally formed after the formation of wirings and TFTs in a pixel portion in manufacturing the active matrix light emitting device.
In the active matrix light emitting device, a switching element formed of a TFT is provided in each pixel, and a driving element for controlling current (current control TFT) is operated by the switching element (switching TFT) to thereby make the EL layer (light emitting layer) emit light. For example, there is a light emitting device described in Japanese Patent Application Laid-open No. Hei 10-189252.
Note that the switching TFT and the current control TFT in the pixel portion require a low off current (Ioff). A structure having a region (LDD region) where a gate electrode does not overlap a low concentration impurity region through a gate insulating film is known as a TFT structure for reducing an off current.
On the contrary, an image circuit for displaying an image and a driver circuit for controlling the image circuit are required to have a high driving capacity (on current: Ion) and prevent deterioration due to a hot carrier effect to thereby improve reliability. A structure having a region (GOLD region) where a gate electrode overlaps a low concentration impurity region through a gate insulating film is known as a structure effective in preventing deterioration of an on current value due to hot carriers.